This invention relates to microcircuits in general and more particularly to an improved method of obtaining profile control in a photoresist using dry processing.
In the making of microcircuits, multilevel processes can have several advantages. Some of these are: the relaxation of constraints on resists to be both sensitive to the exposure medium and resistant to dry etching condition; the planarization of rough surface topography to allow better focusing using optics with small depths of field; and the formation of special resist profiles for lift-off processing. A typical series of steps in a multilevel process includes placing a planarizing layer on a substrate such as silicon covered with the usual thermal SiO.sub.2, depositing a layer of material such as silicon dioxide or silicon nitride over the planarizing layer and then covering that layer with a photoresist. The photoresist is exposed and developed. In the areas in which the photoresist is developed, the silicon dioxide is exposed and then etched away chemically or otherwise. The remaining silicon dioxide then forms a mask permitting removal of the planarizing layer down to the thermal oxide.
Typically dry processing has been used in this process. A process which is referred to as reactive ion etching is described in an article entitled "High Resolution, Steep Profile, Resist Patterns" by J. M. Oran et al., The Bell System Technical Journal, Vol. 58, No. 5 May-June 1979. In the described process, a thick organic layer (a photoresist in the experiments) was covered with an intermediate layer of SiO.sub.2 and a top thin layer of X-ray or photoresist. After exposure and development of the top resist layer, the intermediate layer was etched by CHF.sub.3 ion etching. The thick organic layer was then etched by O.sub.2 reactive ion etching. The authors reported submicron resolution with essentially vertical walls in the thick organic material. An experiment was carried out with a 1.6 micron layer of photoresist serving as the thick organic layer. Reactive RF sputter etching using pure oxygen as the gas forming the plasma and the SiO.sub.2 as the mask was used to etch the organic layer. The RF power density was 0.5 watt/cm.sup.2. It took twenty minutes to complete etching. SEM photographs show perpendicular walls with very little undercut. Typically reactive ion etching is done at less than 50 mTorr pressures with wafers resting on the powered electrode (cathode) at a 13.56 MHz frequency. This results in a slow process. Inverse undercutting has not previously been possible.
An inverse undercut is of particular interest in lift off processes, i.e., when trying to lift off the photoresist after putting down conductors. Preferably good separation between the edge of the deposited conductor and the remaining resist will have been established to permit ease of lift off. Otherwise, lift off becomes more difficult. (Previously this has typically been done using wet chemicals some of which are expensive and carcinogenic.)
Thus, it is an object of the present invention to provide an improved method of processing microcircuits which permits, preferably with dry processing, obtaining steep side walls and even an effective inverse undercut in the photoresist.